Mechanical properties of plywood glued with the use of thermoplastic film of primary low density polyethylene

The plywood industry still most often uses phenol-formaldehyde (РF) and urea-formaldehyde (UF) resins. In Ukraine, 55–60 % of plywood products are made using thermosetting UF resins, and 40–45 % of plywood products are made using - PF resins. However, their main disadvantage is formaldehyde emission. In addition, formaldehyde is emitted during the operation of plywood and other wood-based composite materials. This study is focused on the use of thermoplastic films in plywood technology as an alternative to replacing liquid thermoreactive and toxic adhesives. In this paper, the use of thermoplastic films of primary low-density polyethylene (LDPE) for gluing veneer in the production of plywood is offered. The aim of the study was to determine the mechanical properties of plywood, such as shear strength, bending strength and modulus of elasticity, made of peeled veneer of different wood species bonded with primary LDPE film of different thicknesses. For a better understanding of the gluing process of veneer with a thermoplastic film, different wood species were proposed – poplar, birch, hornbeam and beech, as well as different thicknesses of the film - 50, 80, 100, and 150 µm. The obtained results were compared with the properties of plywood glued with UF adhesive. The quality of bonding was evaluated by the shear strength value, which was determined in accordance with the EN 314-1 standard. The results of this study showed that the plywood samples made from veneer of various wood species and glued with primary LDPE thermoplastic film have satisfactory mechanical properties. It was found that the shear strength of plywood made from poplar, birch, hornbeam and beech veneer and glued with a primary LDPE thermoplastic film of different thicknesses meets the EN 314-1 standard for bonding class 1: dry conditions. It was also found that the wood species and thickness of the film significantly (p ≤ 0.05) affects the mechanical properties of plywood. The plywood made from poplar veneer had the lowest values of bending strength and modulus of elasticity, in addition to the shear strength values, which were at the same level as for birch and hornbeam plywood. Beech plywood samples had the best mechanical properties. Increasing the thickness of the LDPE film improved the mechanical properties of the wood composite. The values of mechanical properties meet the requirements of appropriate standards and confirm the suitability of using the thermoplastic film as adhesive instead of conventional UF adhesive for manufacture of plywood and its application under dry conditions. The production of plywood using thermoplastic films as an adhesive in comparison with the production of plywood glued with UF adhesive has a number of advantages due to the absence of: operations for preparing liquid glue; equipment for applying liquid glue to the veneer surface; washing the technological line for preparation and supply of glue; the need to clean equipment for applying glue; the need for disposal of waste resins and adhesives; emission of harmful fumes of formaldehyde and ammonia. However, further study should be done to solve the problem of poor compatibility of wood and LDPE film, which will help to expand their scope of application.